The present invention pertains generally to the field of air leak detectors for sensing the presence of air bubbles in a fluid. Devices of this general type find an important use in the medical field where they are used for detecting the presence of air bubbles in a tube carrying blood. In many medical and surgical procedures, it is necessary for blood to be transported through tubing outside the patient's body. Examples include the connection of a patient to a heart-lung machine during surgery, or to an artificial kidney machine during hemodialysis.
In the case of hemodialysis, blood from the artery of a patient, usually in his arm, is conveyed through tubing to a dialyzer for purification by dialysis, and then through additional tubing back to the patient's vein. In addition to the dialyzer itself, the blood flow path may include additional elements such as a pump and a flow meter. The development of an air leak anywhere in the external blood flow path could be fatal to the patient, if a large air bubble is allowed to return with the blood to the patient's body. To prevent this a drip chamber is usually connected in the blood flow path to remove any air or other undissolved bubbles in the blood. Additionally, it is desirable to provide some type of air leak detector, preferably at the end of the blood flow path just before it returns to the patient's vein.
One type of prior art safety device intended to prevent return of air to the patient's blood comprises a device to sense the level of blood in the drip chamber. Such devices operate on the theory that so long as the blood level is high enough, no air will be drawn into the return line. However, this type of prior art device can be confused by the presence of foam, and it also suffers from the disadvantage of providing only indirect sensing, as it does not actually sense air bubbles in the return line directly to the patient.
Other types of prior art devices have been designed to sense the presence of an air bubble, and to provide an alarm signal in response thereto. Some of these devices operate by sensing the difference in optical properties between blood and air. Others operate by sensing differences in magnetic or ultrasonic properties of blood and an air bubble. One problem existing with these prior art devices is that they are subject to nuisance alarms. Because either a larger air bubble or a number of small air bubbles can present a dangerous situation to the patient, these prior art devices have been designed with high enough sensitivity to detect small bubbles. However, a few bubbles by themselves, if small enough, can be safely tolerated by the patient, but the prior art sensors will activate their alarms even for a single small bubble. Since these commonly occur from time to time, but represent no real threat to the patient, a number of unnecessary interruptions to the medical procedure result. The prior art systems have not been able to reduce their sensitivity to avoid triggering on small bubbles, since a larger number of small bubbles represents a dangerous condition which must be sensed.
Another problem with prior art air leak sensors is that while they may detect bubbles in blood, in general they are unable to detect air bubbles in the saline solution passed through the tubing during start-up procedures in hemodialysis. Accordingly, these devices provide no protection during the initial phases of a hemodialysis procedure.
The present invention provides an air leak detector which responds not only to the presence of an air bubble, but also to the total volume of the air bubbles detected in a given period of time. Thus, the present invention will trigger its alarm if a large air bubble or column should occur. It will also detect the presence of a small air bubble but instead of triggering its alarm, it will accumulate and keep track of the various volumes of bubbles detected. If the accumulated value exceeds the safe preset value during a given time period, the alarm will be activated.
It is also important that the air leak detecting device itself should be fail-safe in operation, so that a failure on the part of the detector will not put the patient in jeopardy. The present invention accomplishes this goal by providing a spring-loaded clamp for pinching off the blood tube and at the same time physically opening the switch for the blood pump. In normal operation, the clamp is held open by an electromagnet, to be released when an out-of-tolerance quantity of air is detected. However, should the power supply to the detector fail or should some of the circuitry in the detector itself fail, the cutting of power will result in stopping the blood flow to warn of inoperativeness of the detector, rather than allowing the procedure to continue without this important protection.
The present invention senses optical properties of the blood or other fluid in order to detect air bubbles or clots. The detector automatically and continuously adjusts itself for varying optical densities of the fluid being monitored. For example, the hemodialysis process is started with the dialyzer and external blood flow path filled with a saline solution. As the hemodialysis process gets underway, this saline solution is gradually replaced by the patient's blood flowing through the path. The tube carrying fluid to the patient's vein therefore initally contains saline solution, which is clear, then later contains blood which is optically much denser. Automatic circuitry within the present detector changes the operating point of the optical sensing means to accommodate the changing density of the fluid.
In addition, special circuitry enables the detector to respond to bubbles or objects in the fluid which have either greater or lessor light transmitting ability than the fluid. Thus, the detector responds to air bubbles in the saline solution, to air bubbles in blood, or to clots in the blood.
Another feature of the present invention allows it to respond not only to a large bubble or column of air in the tube, representing a gross leak, but also to a series of small micro bubbles. Micro bubbles above a very small minimum threshold size are measured and integrated over a period of time. When a predetermined volume of micro bubbles has been detected, the detector will so indicate. The sensitivity of the detector to different quantities of micro bubbles can easily be selected.